Damp measurement

[John_5040782]

Morning all - I know that there have been several threads on this topic over the last few months so I hope I am not boring everyone if I raise it again!

1. When applying the meter probes to the surface of the internal lining panel does it really indicate the dampness of the structure underneath?

2. I have been told that a small area of the outer wall in the shower compartment gives a reading of 25% and that it should be re-checked after 3 months. My own B&Q meter also indicates moisture. Note that it is several weeks since we used the shower and then only on 2 or 3 occasions.

3. So the question arises as to how accurately that surface measurement reflects the true moisture within the wall?

4. Presumably the dealer is expecting the moisture to evaporate through the inner lining (it certainly won't evaporate through the outer aluminium sheet).

5. Does anyone have thoughts on this matter? How much timber is there in the outer wall of a 1998 Abbey? Is timber only used around the window and door openings, and presumably at the panel edges, or is there some elsewhere in the body of the construction?

6. Is it possible to speed up the drying out process in any way?

7. Over to you!

 

[ProfJohnL]

Hello John,

Your posting has highlighted some of the many uncertainties with using 'damp' meters.

I will try and answer some of your questions, but I fear it will leave you with even more questions.

These handheld meters all work on the principal of measuring the electrical resistance between the probes on the device.

When touch a surface with the probes, depending on the materials of the surface and any included moisture a conductive path is created between the probes. How conductive depend on the materials and any moisture or other chemicals there may be present, on or in the material

Electricity is very lazy, it will always find the route of least resistance, and if there are a number of parallel routes (different layers in the material) the current will be shared between them but primarily going for the easiest.

Unfortunately what you do not know is which layers are the most conductive, so you cannot say the apparent dampness is in particular layer.

Another issue is how far in do you have to push the probes? Consider, if you have a plastic laminate on the surface, then just touching the surface without piercing it is likely to give a no damp reading, because the laminate surface is usual water resistant.

However by piercing the top surface and contacting the probes with the substrate, which is usually a fibrous composite, you may well find a significant reading.

If you then push harder and embed the probe properly in the substrate you may well find that you get a greater detection of dampness. This is because the a greater surface area of the probes is now in contact with the material, and that increases the chances of finding a conductive route.

Also consider if the surface of the wall may be contaminated with anything such as cleaners that may have left a residue which is conductive.

As you can see there are many factors that will affect the reading you get. Is the instrument accurate? When was it last calibrated and what correction factors have been established to convert the meter reading into accurate figures?

Interpreting the reading is also important, because different materials have different normal levels of moisture within them. Do you know what is a normal moisture range for the surface you are checking? Then is 25% abnormal? How does it change with the ambient relative humidity and temperature?

Sad to say there are some rouge dealers who know how to make a meter give a worse figure than is correct as it might generate some work for them. This is easy because most of the public don't know how the meters work and what a good reading should be.

As for any dampness in a wall, If the reading is fairly consistent across the whole of the wall , then it is more likely to be normal moisture levels.

If the reading is markedly higher in a smaller region them it could be water ingress or retention.

As you correctly surmise, to remove the excess moisture it wil only be able to leave via the internal wall.

To reduce dampness, you must reseal the source of the water ingress, and then ensure the inside of the caravan is properly ventilated especially around the affected area. Sometimes a dehumidifier can help

The construction of the caravan will use wood in a number of places that may not be visible or obvious, the amount will depend on the specific model of van you have.

 

[John_5040782]

Thanks John L for your excellent reply.

My thoughts at the moment are to find a cheap dehumidifier on Ebay and then if the indicated moisture level decreases find a way of increasing the water resistance of the lining board. A surface coating or even a shower curtain against the outer wall?

regards - John

 

[Happy-Lappy]

John, on our caravan the shower curtain is always used, as this wraps around 3/4 of the shower cubicle. The only exposed area is the white Grp plastic walling, inwhich houses the sink and bathroom cupboard,

best regards, Martin

 

[paul_5132583]

John, after spending most of last winter resealing and renewing all of the wall board, i have found that if you have damp the only way to repair is to firstly stop the water coming back in, resealing every external joint in my case, window rubbers awning rails etc. I don't believe that you can dry a damp patch without removing either all of the wall board or at the very least the vinyl protective layer, i originally spent many days and nights with a dehumifier and then a warm air blower, with hardly any change, the moisture is simply stuck between the aluminium and the vinyl sheeting, when its cold outside the saturated wall is made worse via the cold brigde between external cold and internal warm surfaces. With regard to the damp meter, any moisture level over 20% is actively rotting the timbers within your caravan, and the dry areas will act as a sponge, capillary action i think it called.

Regards Paul.

 

[John_5040782]

Hi Paul - You've given me food for thought! When you talk of the vinyl protective layer, are you saying that there is a thin film on top of the decorative wall board? If so does this vinyl peel off leaving the decorative surface underneath undamaged?

Thanks for your help - John

 

[paul_5132583]

Hi John, afraid that there isn't a protective layer to peal off, i would imagine that it is very similiar to vinyl wall paper,i have read that a cheaper repair is to ply line the wall and paper using such a finish.

I would say that the paper peals very easily if the wall board is damp, before it eventually breaks down and becomes soft, the softness is the ply lining of the wallboard breaking down.

Peel the paper off if you can and you will dry the wall overnight, it can be that quick, from the moment the paper is removed if the area is severe you will be quite surprised at the strong smell of damp, which wasn't evident before, however i as say, dry over night and the smell will be gone, ready for reboarding.

Being rather cynical i would say from experience that most older caravans are damp some sooner than other's, and some on this forum less than a year old! however it dosn't need to be terminal, and a lot of owners are totally unaware.

Regards Paul.

 

[John_5040782]

Thanks for the input from John-L, Happy-Lappy & Paul - I am most appreciative. Paul commented "a lot of owners are totally unaware" and perhaps ignorance is bliss. Is it a matter of information overload, it could be compared to cars where we used to have oil pressure gauges, ammeters etc. now we just have lights that comes on if there is a problem. Perhaps I should wait for obvious physical signs of a problem and then take remedial action. Are manufacturers being optimistic expecting wall board to be a satisfactory lining for a shower compartment? Happy-Lappy's shower room with a curtain around 3/4 of the compartment may be the best solution.

Thanks to all - John

 

[RayS]

It's many years since I was involved in "moisture" measurment for applications in grain drying and moisture in clay before extrusion into bricks etc. but this thread reminds me of the difficulties faced.

First, I think the "resistance" mentioned in john L's post should be "impedence" i.e. measured with an alternating current rather than direct current, as the latter tend to polarise very quickly.

Secondly, the area of probe in contact with the sample and the quality of this connection are critical, and so with the simple hand held meters it it quite possible to get a change of reading by just pressing harder.

Because of the way the popular meters are constructed it is quite possible to get a "high" reading if the surface is moist e.g. with condensation, rather than the substructure. just Try a reading on your forearm and then wipe the skin with a damp cloth.

As with most analytical measurements, the sample presentation and preparation is most important and can influence the results considerably, so please do be aware of dealer claims to have found "damp" when part exchanging. Ask to see their meter and when it was last calibrated ( and by who).

I've been out of the measurment business for a few years now and there may well be better devices available - perhaps others out there will have more up to date knowledge ( rob-jax ?)

But I still have occaisional nightmares about the problems of moisture content, humidity, relative humidity, dew point measurements particularly for the production enviorment of pharmaceuticals.

 

[ProfJohnL]

It's many years since I was involved in "moisture" measurment for applications in grain drying and moisture in clay before extrusion into bricks etc. but this thread reminds me of the difficulties faced.

First, I think the "resistance" mentioned in john L's post should be "impedence" i.e. measured with an alternating current rather than direct current, as the latter tend to polarise very quickly.

Secondly, the area of probe in contact with the sample and the quality of this connection are critical, and so with the simple hand held meters it it quite possible to get a change of reading by just pressing harder.

Because of the way the popular meters are constructed it is quite possible to get a "high" reading if the surface is moist e.g. with condensation, rather than the substructure. just Try a reading on your forearm and then wipe the skin with a damp cloth.

As with most analytical measurements, the sample presentation and preparation is most important and can influence the results considerably, so please do be aware of dealer claims to have found "damp" when part exchanging. Ask to see their meter and when it was last calibrated ( and by who).

I've been out of the measurment business for a few years now and there may well be better devices available - perhaps others out there will have more up to date knowledge ( rob-jax ?)

But I still have occaisional nightmares about the problems of moisture content, humidity, relative humidity, dew point measurements particularly for the production enviorment of pharmaceuticals.

Quite correct Ray, alternating current is the better way, but a number of the cheaper models available for DIY only use DC, regardless of the type of current it is fundamentally the conductive path(s) that is measured.

 

[John_5040782]

Thanks Ray for your input. As a matter of interest I did wipe the surface with meths and allowed it to dry before taking another reading - there was no significant change. So I decided to remove the window rubber seal along the bottom of the adjacent window opening and examine the timber underneath. Firstly the timber appears to be anti-rot treated and and was totally dry according to the damp meter, which has gone a long way to easing my concern. So while the weather is so good and dry I have left the rubber seal adrift for the moment to see whether there is any change in the "problem" areas. I'll keep you all informed on developements - regards - John